Photovoltaic (PV) devices are PV cells or PV modules containing a plurality of PV cells or any device that converts photo-radiation or light into electricity. Generally, a thin film PV device includes two conductive electrodes sandwiching a series of semiconductor layers. The semiconductor layers include an n-type window layer in close proximity to a p-type absorber layer to form a p-n junction. During operation, light passes through the window layer, and is absorbed by the absorber layer. The absorber layer produces photo-generated electron-hole pairs, the movement of which, promoted by an electric field generated at the p-n junction, produces electric current that can be output through the two electrodes.
Since light has to pass through the window layer to be converted to electricity, it is desirable to have a thin window layer that allows the most amount of light to pass therethrough. The more light that passes through the window layer, the more efficient the device. Thus, one method that has been used to increase device photo-conversion efficiency is to use a window layer that is as thin as possible, while maintaining the p-n junction with the absorber layer.
Another method that has been used to enhance device photo-conversion efficiency is to subject the absorber layer to a cadmium chloride (CdCl2) activation treatment. Alternative compounds for the activation treatment can also be used such as, for example, NHCl2, ZnCl2, TeCl2, or other halide salts.
The CdCl2 activation treatment increases the grain size and reduces defect areas in the absorber layer. Specifically, one factor that may limit thin-film photo-conversion efficiency is the number of photo-generated electron-hole pairs (i.e., carriers) that are trapped and then recombined before they are output as electricity by the device. In some instances, carriers may get trapped at structural defects such as defective grain boundaries within various layers of the device. For example, the semiconductor absorber layer is formed of grains, also known as crystallites. Crystallites are small, microscopic crystals, where the orientation of the crystal lattice within the crystallite is the same. But, a defect exists where the orientation of the crystal lattice changes from one grain to another. Hence, the crystallites that make up the absorber layer may be said to have defective grain boundaries where crystallites on each side of the boundary are identical, except in crystal orientation.
In any case, the larger the grains that make up the absorber layer, the lesser the number of grain boundaries present in the absorber layer. Thus, the CdCl2 activation treatment increases the size of the grains or crystals that make up the absorber layer and thus reduces the number of grain boundaries available to trap carriers. Device efficiency may thus be enhanced.
For example, a thin-film PV device may have a window layer formed of cadmium sulfide (CdS) and an absorber layer formed of cadmium telluride (CdTe). The CdCl2 activation treatment includes applying CdCl2, for example, in liquid or vapor form, to the CdTe of the absorber layer, and then annealing the absorber layer at a particular anneal temperature, for example about 400° C. to about 420° C., for a particular anneal time, for example, from about 10 minutes to about one hour. The anneal temperature is generally high enough and the anneal time long enough to promote recrystallization of the CdTe crystallites.
The recrystallization of the cadmium telluride can take two forms or a combination of the following two forms: (1) intragrain or primary recrystallization (recrystallization that changes crystallite grain orientation); and (2) intergrain or secondary recrystallization (recrystallization resulting from grain coalescence).
The primary crystallization leads to adjacent grains, which were oriented differently, to now be oriented in the same direction. Hence, primary crystallization results in a lesser number of defective boundaries being available to trap carriers therein. By contrast, the secondary recrystallization results in grain growth as smaller grains coalesce into larger ones. Thus, it too, leads to a reduced number of grain boundaries, which could contain defects.
Further, in addition to reducing the number of defective grain boundaries in the absorber layer, the CdCl2 activation treatment also repairs some of the defects in the grain boundaries. This is done through the incorporation of chlorine atoms (or ions) from the CdCl2 into the CdTe absorber layer. Other mechanisms believed to repair or passivate such defects include the formation of doping complexes within the absorber layer created by cadmium vacancies, the incorporation of chlorine atoms to occupy tellurium sites, and inter-diffusion of materials between the absorber layer and the semiconductor window layer.
The CdCl2 and the heat from the CdCl2 activation treatment, while being beneficial in reducing the number of defective grain boundaries in the absorber layer may also promote chemical fluxing. Fluxing occurs when a chemical element from one layer of a photovoltaic device under fabrication, where it is in high concentration, flows into another layer where there is a low concentration, or where it is not.
In this case, the CdCl2 activation treatment may increase the mobility of sulfur atoms from a CdS window layer causing it to diffuse into a CdTe absorber layer. This fluxing of sulfur can consume the CdS in the window layer, overly thinning the layer and, in extreme situations, entirely removing it in some areas resulting in areas where the p-n junction is degraded or lost. The total removal of the CdS window layer may be exacerbated in devices where a very thin window layer was initially used to enhance device efficiency.
It would be desirable, therefore, to provide a technique for reducing or controlling the amount of sulfur fluxing from the window layer into the absorber layer during a CdCl2 activation treatment.